The present invention relates to a system for controlling the ignition timing of an automotive engine at the failure of a sensor, and more particularly to an ignition timing control system using a learning control system which is provided for updating data stored in a table for controlling the fuel supply in an electronic fuel-injection system.
In one type of electronic fuel-injection control, the amount of fuel to be injected into the engine is determined in accordance with engine operating variables such as mass air flow, engine speed and engine load. The amount of fuel is decided by a fuel injector energization time (injection pulse width). Basic injection pulse width (T.sub.p) can be obtained by the following formula. EQU T.sub.p =K.times.Q/N (1)
where Q is mass air flow, N is engine speed, and K is a constant.
Desired injection pulse width (T.sub.i) is obtained by correcting the basic injection pulse (T.sub.p) with engine operating variables. The following is an example of a formula for computing the desired injection pulse width. EQU T.sub.i =T.sub.p .times.(COEF).times..alpha..times.K.sub.a ( 2)
where COEF is a coefficient obtained by adding various correction or compensation coefficients such as coefficients on coolant temperature, full throttle open, engine load, etc., .alpha. is a .lambda. correcting coefficient (the integral of the feedback signal of an O.sub.2 -sensor provided in an exhaust passage), and K.sub.a is a correcting coefficient by learning (hereinafter called learning control coefficient). Coefficients, such as coolant temperature coefficient and engine load, are obtained by looking up tables in accordance with sensed informations. The value of the learning control coefficient K.sub.a is obtained from a K.sub.a -table in accordance with engine load.
On the other hand, the ignition timing of the engine is decided also by the mass air flow Q. More particularly, if the mass air flow Q increases, the amount of fuel increases and, at the same time, ignition timing is advanced as increasing of the fuel. Accordingly, if a mass air flow meter deteriorates to fail to produce a proper output voltage, the air-fuel ratio of mixture supplied to the engine deviates from a stoichiometry and improper ignition timing is set. For example, if the output voltage increases by the failure of the mass air flow meter, the ignition timing is advanced regardless of engine operating conditions. Such improper advancing of timing will occur the knocking of the engine.